Alzabo is a program and a suite of modules, with two core functions.
Its first use is as a data modelling tool. Through either a schema
creation interface or a perl program, you can create a set of schema,
table, column, etc. objects to represent your data model. Alzabo is
also capable of reverse engineering your data model from an existing
system.

Its second function is as an RDBMS to object mapping system. Once you
have created a schema, you can use the
Alzabo::Runtime::Table and
Alzabo::Runtime::Row classes to access its
data. These classes offer a high level interface to common operations
such as SQL SELECT, INSERT, DELETE, and UPDATE commands.

A higher level interface can be created through the use of the
Alzabo::MethodMaker module. This module
takes a schema object and auto-generates useful methods based on the
tables, columns, and relationships it finds in the module. The code
is generates can be integrated with your own code quite easily.

To take it a step further, you could then aggregate a set of rows from
different tables into a larger container object which could understand
the logical relationship between these tables.

The Alzabo::Runtime::Row objects support
the use of a caching system. Caching modules are included with the
distribution. However, you may substitute any caching system you like
provided it has the appropriate method interface.

Alzabo has a lot of documentation. If you are primarily interested in
using Alzabo as an RDBMS-OO wrapper, much of the documentation can be
skipped. This assumes that you will create your schema via the schema
creation interface or via reverse engineering.

the Alzabo::Runtime::Row manpage - The row objects are how data is updated,
deleted, and retrieved from the database. Its also important to read
the section on its import, as
this is used to determine how caching is done (for now).

the Alzabo::MethodMaker manpage - One of the most useful parts of Alzabo. This
module can be used to auto-generate methods based on the structure of
your schema.

the Alzabo::ObjectCache manpage - This describes how to select the caching
modules you want to use. It contains a number of scenarios and
descibes how they are affected by caching. If you plan on using
Alzabo in a multi-process environment (such as mod_perl) this is very
important.

Every schema keeps track of whether it has been instantiated or not.
A schema that is instantiated is one that exists in an RDBMS backend.
This can be done explicitly by calling the schema's
create method. It is also
implicitly set when a schema is created as the result of reverse engineering.

Instantiation has several effects. The most important part of this is
to realize that once a schema is instantiated, the way it generates
SQL for itself changes. Before it is instantiated, if you ask it to
generate SQL via
Alzabo::Create::Schema->make_sql,
it will generate the set SQL statements that are needed to create the
schema in the RDBMS.

After is instantiated, the schema will instead generate the SQL
necessary to convert the version in the RDBMS backend to match the
object's current state. This can be though of as a SQL 'diff'.

While this feature is quite useful, it can be confusing too. The most
surprising aspect of this is that if you create a schema via reverse engineering and then call
Alzabo::Create::Schema->make_sql,
you will not get any SQL. This is because the schema knows that it is
instantiated and it also knows that it is the same as the version in
the RDBMS, so no SQL is necessary.

In Alzabo, data is returned in the form of a row object. This object can be used to access the
data for an individual row.

Unless you are retrieving a row via a unique identifier (usually its
primary key), you will be given a cursor
object. This is quite similar to how DBI uses statement handles
and is done for similar reasons.

The first thing you'll want to do is create a schema. The easiest way
to do this is via the included web based schema creation interface,
which requires the HTML::Mason package from CPAN (www.cpan.org) to
run.

This interface can be installed during the normal installation
process, and you will be prompted as to whether or not you want to use
it.

The other way to create a schema is via a perl script. Here's the
beginning of such a script:

Alzabo uses exceptions as its error reporting mechanism. This means
that pretty much all calls to its methods should be wrapped in
eval{}. This is less onerous than it sounds. In general, there's
no reason not to wrap all of your calls in one eval, rather than each
one in a seperate eval. Then at the end of the block simply check the
value of $@. See the code of the included HTML::Mason based
interface for examples.

Also see the Alzabo::Exceptions documentation,
which lists all of the different exception used by Alzabo.

Alzabo is a powerful tool but as with many powerful tools it can also
be a bit overwhelming at first. The easiest way to understand some of
its basic capabilities is through some examples.. Let's first assume
that you've created the following schema:

First of all, let's do something simple. Let's assume I have a
person_id value and I want to find all the movies that they were in
and print the title, year of release, and the job they did in the
movie. Here's what it looks like:

# all the rows in the credit table that have the person_id of 42.
my $cursor = $person->rows_by_foreign_key( foreign_key => $person_t->foreign_keys_by_table($credit_t) );

print $person->select('name'), " was in the following films:\n\n";
while (my $credit = $cursor->next_row)
{
# rows_by_foreign_key returns a RowCursor object. We immediately
# call its next_row method, knowing it will only have one row (if
# it doesn't then our referential integrity is in trouble!)
my $movie =
$credit->rows_by_foreign_key( foreign_key => $credit_t->foreign_keys_by_table($movie_t) )->next_row;

Let's assume that we've been passed a hash of values representing an
update to the location table. Here's a way of making sure that that
this update won't lead to a loop in terms of the parent/child
relationships.

Each subclass of Alzabo::SQLMaker is capable of exporting functions
that allow you to use all the SQL functions that your RDBMS provides.
These functions are normal Perl functions. They take as argument
normal scalars (strings and numbers), Alzabo::Column objects, or
the return value of another SQL function. They may be used to select
data via the
Alzabo::Runtime::Table->function
method. They may also be used as part updates, inserts, and where
clauses, any place that is appropriate.

In MySQL, there are a number of various types of integers. The type
TINYINT can hold values from -128 to 127. But what if have more than
127 movies? And if that's the case we might have more than 127 people
too.

For safety's sake, it might be best to make all of the primary key
integer columns INT columns instead. And while we're at it we want to
make them UNSIGNED as well, as we don't need to insert negative
numbers into these columns.

Alzabo aims to be as cross-platform as possible. To that end, RDBMS
specific operations are contained in several module hierarchies.

The first, the Alzabo::Driver::* hierarchy, is used to handle
communication with the database. It uses DBI and the appropriate
DBD::* module to handle communications. It provides a higher level
of abstraction than DBI, requiring that the RDBMS specific modules
implement methods to do such things as create databases or return the
next value in a sequence.

The second, the Alzabo::RDBMSRules::* hierarchy, is used during
schema creation in order to validate user input such as schema and
table names. It also generates SQL to create the database or turn one
schema into another (sort of a SQL diff). Finally, it also handles
reverse engineering an existing database.

The this, the Alzabo::SQLMaker::* hierarchy, is used to generate
SQL and handle bound parameters for select, insert, update, and delete
operations.

The RDBMS to be used is specified when creating the schema.
Currently, there is no easy way to convert a schema from one RDBMS to
another, though this is a future goal.

First, reverse engineering does not handle constraints (including
foreign keys). This will change in the future.

Second, reverse engineering cannot determine from the existence of a
sequence that the sequence is meant to be used for a particular column
unless the sequence was created as a result of assigning the serial
type to a column.

Third, there is no support for large objects. This was considered but
given that 7.1 now supports rows larger than 32K it was determined
that supporting large objects was not worth the amount of effort
reqiured.

There are objects representing the schema, which contains table
objects. Table objects contain column, foreign key, and index
objects. Column objects contain column definition objects. A single
column definition may be shared by multiple columns, but has only one
owner.

Note that more than one column _may_ share a single definition object
(this is explained in the
Alzabo::Create::ColumnDefinition
documentation). This is only relevant if you are writing a schema
creation interface.

Alzabo::Driver
These objects handle all the actual communication with the database,
using a thin wrapper over DBI. The subclasses are used to implement
functionality that must be handled uniquely for a given RDBMS, such as
creating new values for sequenced columns.

Alzabo::SQLMaker
These objects handle the generation of all SQL for runtime operations.
The subclasses are used to implement functionality that varies between
RDBMS's, such as outer joins.

Alzabo::RDBMSRules
These objects perform several funtions. First, they validate things
such as schema or table names, column type and length, etc.

Second they are used to generate SQL for creating and updating the
database and its tables.

And finally, they also handle the reverse engineering of an existing
database.

Alzabo::Config
This class is generated by Makefile.PL during installation and
contains information such as what directory contains saved schemas and
other configuration information.

Alzabo::ChangeTracker
This object provides a method for an object to register a series to
backout from multiple changes. This is done by providing the
ChangeTracker object with a callback after a change is succesfully
made to an object or objects. If a future change in a set of
operations fail, the tracker can be told to back the changes out. This
is used primarily in
Alzabo::Create::Schema.

Alzabo::MethodMaker
This module can auto-generate useful methods for you schema, table,
and row objects based on the structure of your schema.

In some environments (mod_perl) we would like to optimize for memory.
For an application that uses an existing schema, all we need is to be
able read object information, rather than needing to change the
schema's definition. This means there is no reason to have the
overhead of compiling all the methods used when creating and modifying
objects.

In other environments (for example, when running as a separately
spawned CGI process) compile time is important.

For most people using Alzabo, they will use one of the existing schema
creation interfaces and then write an application using that schema.
At the simplest level, they would only need to learn how to
instantiate Alzabo::Runtime::Row objects and how that class's methods
work. For more sophisticated users, they can still avoid having to
ever look at documentation on methods that alter the schema and its
contained objects.